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Fuel pressure woes.
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TOPIC: Fuel pressure woes.

Re: Fuel pressure woes. 3 weeks, 1 day ago #83436

  • rich99
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Hi, I am reviving and adding to this thread.

My son, Justin, runs his 87 SE30 in Norcal. Primarily at Sonoma, Thunderhill and occasionally Laguna Seca. For some time we have suspected a loss of power at 4500 rpm and higher. Like a hesitation or lag when the pedal is down. This was confirmed by a veteran we asked to drive the car for a few laps at Sonoma. So, we replaced the in-tank pump. Later installed a new 3 bar FPR. Note: Also, car does have the AIM MXL2 with fuel pressure sensor and sampling port installed. At idle we saw ~37 psi and when we mashed the throttle it dropped to 33-35. Now, we just replaced the in-line pump with a new Bosch unit. Judging by the looks of what we removed I'd say it was the original, so 30 years old. Buttoned everything up, started it up and pressure at idle was ~39, more important, mashing throttle now it spikes to 44. Looking forward to what this means on the track. Will report back how things go next trip to Sonoma. Finders crossed.

PS: In off season injectors are coming out for professional bench cleaning and testing. I use a guy in Sacramento (Dr Injector) who does great work and yes, he can check them for leaks in the body and tip.

Re: Fuel pressure woes. 3 weeks ago #83437

  • Ranger
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You made two references to FP changing at WOT. In both cases the engine had a load on it, correct? I mean as opposed to a car being in neutral and someone playing with the gas pedal and watching FP.

Imo, everyone should have a fuel pressure sensor on their dash. This is hugely helpful when one has an engine management problem because it rules out FP as a possible cause.

Don't get your hopes up. The DME will happily change injector duration to account for low FP. 10psi should be within it's range to adjust to.

The other gauge you need is a fuel to air ratio meter. That will tell you if something funny is going on with your mixture. For example, biggest engine management culprit is the AFM and it can cause the engine to run lean. When my AFM punked me years ago, mostly because well meaning folks felt that they knew how to adjust the AFM spring, my engine would often start missing between 4500-5200rpm at WOT.

My then brand new F/A meter was telling me that the engine was running very lean, but because the meter was new, I didn't trust it. I'd have spared myself a lot of pain had I trusted what it was trying to tell me.
"Tower, this is Ranger requesting a flyby.
"Negative Ranger the pattern is full."
Last Edit: 3 weeks ago by Ranger.

Re: Fuel pressure woes. 3 weeks ago #83438

  • rich99
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Hello Ranger!

Thank you for responding. Both FP readings I mentioned were under static conditions. Car at the shop. in neutral, engine not under load. No doubt The true test is on track.
We need to pull up past FP data from Sonoma runs. Then go back out to the track and collect similar data (now with new fuel pump) and overlay/compare to see what it reveals. Should have that completed this month.

You're spot on recommending installing Air/Fuel meter. AIM does have an add on module for $380 (LCU-One) for that purpose. Will consider it.

I take it you eventually ended up replacing your AFM?

Reason I ask is our AFM was also "adjusted/tweaked/tuned" at a shop.

Our dynojet data from 3 pulls in April 2017 showed A/F at 4642 RPM as 12.29,12.36 and 12.44.

Re: Fuel pressure woes. 3 weeks ago #83439

  • Ranger
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A no load test isn't very meaningful. With no load, fuel demand is low, air demand is low, and vacuum for both idle and WOT is high. If engine was under load, vac would be low at WOT and FPR would react to that by increasing FP. As a result of low fuel demand, nothing that has anything to do with fuel is really tested in a low engine load test. As a result of low air demand, the AFM isn't tested. Because no load means no vac change no matter what you do with the throttle, the FPR's reaction to changing vac isn't tested.

$380 is an awful lot for a F/A meter. Prob get a gauge on your dash and a wideband O2 sensor for 1/2 that cost. Have to weld a bung on to your exhaust. Note that you should connect the gauge's data out to your AIM.
"Tower, this is Ranger requesting a flyby.
"Negative Ranger the pattern is full."
Last Edit: 3 weeks ago by Ranger.

Re: Fuel pressure woes. 1 week, 6 days ago #83440

  • rich99
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Reporting back after a day at Sonoma. Changing the FPR and external pump made a HUGE difference. The AIM data (FP before vs after) plots overlaid (real time at the track) clearly showed it. Most important, he set a new best lap time 2 seconds faster. PUMPED....literally
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